A major part of the debate around shale gas has focused on methane emissions and climate change. Natural gas, when burned, produces half as much CO2 as coal, so switching from coal fired power stations (still ~40% of our electricity supply) to natural gas could provide substantial emissions reductions. However, if a portion of the methane produced during extraction is allowed to leak into the atmosphere, then this could offset the CO2 gains, because methane is itself a powerful greenhouse gas.
We've seen a number of papers in recent times trying to get a handle on what proportion of methane produced from shale wells is ending up in the atmosphere, rather than in our boilers. Direct measurements at drilling sites have given low estimates, but regional overflight measurements have indicated that overall estimates might be underestimated - although with regional overflights, the source of methane (conventional gas, coal mining, shale gas, agricultural) cannot be determined.
The key question to ask is - how relevant are these estimates to global climate change, and are we seeing changes in methane emission rates impacting on the global concentration of methane in our atmosphere, and therefore on global climate change?
Who better to answer this question than the climate experts at RealClimate.org - certainly no stooges for the oil and gas industry.
So - are reported increases in methane emissions bad news for global warming? (my emphasis added)
Not really, because the one real hard fact that we know about atmospheric methane is that it’s concentration isn’t rising very quickly. Methane is a short-lived gas in the atmosphere, so to make it rise, the emission flux has to continually increase. This is in contrast to CO2, which accumulates in the atmosphere/ocean system, meaning that steady (non-rising) emissions still lead to a rising atmospheric concentration. There is enough uncertainty in the methane budget that tweaks of a few percent here and there don’t upset the apple cart. Since the methane concentration wasn’t rising all that much, its sources, uncertain as they are, have been mostly balanced by sinks, also uncertain. If anything, the paper is good news for people concerned about global warming, because it gives us something to fix.Also, in more general terms:
The US is apparently emitting more than we thought we were, maybe 30 Tg CH4 per year. But these fluxes are relatively small compared to the global emission rate of about 600 Tg CH4 per year. The Arctic and US anthropogenic are each about 5% of the total. Changes in the atmospheric concentration scale more-or-less with changes in the chronic emission flux, so unless these sources suddenly increase by an order of magnitude or more, they won’t dominate the atmospheric concentration of methane, or its climate impact.I am not a climate scientist, and there are probably too many non-experts shouting their views from the rooftops, so I will pose my conclusions instead as a tentative question: are we overestimating the importance of shale gas methane emissions with respect to climate change?